Microgels that are sub-micron in size are also known as nanospheres.
The properties of microgels make them particularly useful in a wide range of applications, such as in additives, in advanced material formulations for foams or fibers, in binders and re-dispersible latexes.
Diblock (A-B) and triblock (A-B-A) copolymers typically form micelles in solvents where one component of the block is soluble (miscible) and the other less soluble or insoluble (immiscible). Many such examples are known, such as a block copolymer having a solvophobic block of polystyrene and a hydrophilic block of poly(ethylene oxide) which forms micelles in alcoholic solvents. The solvophilic-solvophobic balance affects the propensity of the block copolymer to form micelles. Micellization can sometimes be achieved by first dissolving the block copolymer in a mutual solvent, and then dispersing this solution in a non-solvent for one of the blocks or by changing the ratio of solvent to non-solvent. The size of the resulting micelles depends on the molecular weights of the component blocks and the nature of the solvent.
Micelles are dynamic systems that equilibrate between a micellic and non-micellic form of a block copolymer. In order to “stabilize” a polymeric micelle, it is necessary to chemically modify the formed micelles, stabilizing them into the desired configuration, typically using crosslinking reactions. The micelles can be stabilized either by crosslinking of the core or the shell. Care is required in preparing microgels as the reactive groups present within these systems can undergo intermolecular reactions, which can lead to intractable networks.
Core crosslinked micelles based on block copolymers are disclosed by Nair and Yoo in U.S. Pat. No. 5,429,826. A variety of shell crosslinked micelles based on block copolymers are disclosed by Wooley et al. in WO97/49387, the disclosures of which are incorporated herein by reference. The block copolymers used in these disclosures were synthesized by anionic or group transfer polymerization. These procedures suffer from a number of disadvantages. These procedures are expensive to implement. They are also compatible with only a narrow range of monomers. Protic monomers (e.g. methacrylic acid) must be protected in order for these techniques to be used and then deprotected before micellization. Anionic and group transfer polymerization cannot be generally applied to make copolymer blocks. These considerations have severely limited the range of block copolymers that have been used for the production of micelles.
Crosslinked micelles have also been prepared from block copolymers synthesized by radical polymerization using dithiocarbamate photoiniferters as described by Saito et al. (J. Appl. Polym, Sci., 1997, 63,849) or by using atom transfer polymerization as disclosed by Armes in J. Am. Chem. Soc. 1999, 121, 4288. These methods for block copolymer synthesis also suffer from the very narrow range of monomers to which they are applicable.
Recently new methods of block copolymer synthesis by radical polymerization based on the use of chain transfer agents, which react, by reversible addition-fragmentation chain transfer (RAFT) have been described in WO 98/01478. These reagents provide an economical route to block copolymers derived from a diverse range of monomers. The preferred chain transfer agents for use in these methods include thiocarbonylthio compounds (dithioesters, trithiocarbonates, xanthates and dithiocarbamates) and certain macromonomers. A particular advantage is that acid-containing monomers can be polymerized without the need for protection-deprotection strategies. Reactive functionalities allowing crosslinking may also be incorporated.
The present invention now recognizes that block copolymers produced by RAFT polymerization may be advantageously used in the production of micellar structures avoiding some of the problems associated with prior art processes for forming crosslinked micelles.
The present invention also recognizes that it is useful to encapsulate third party molecules or particles with the particles resulting from the stabilization of the micelles, and also such stabilized micelles, which are microgels, may be post-preparatively modified by chain extending of the arms.
The virtue of the process of this invention is that a much wider range of functionality, for example for crosslinking, for controlling the surface functionality and for determining the microenvironment in the core of the micelle, is possible.